Saharan Dust and Visibility of Venus and Jupiter
September 21, 2016
For the conjunction of Jupiter and Venus on August 28, 2016, I hoped to photograph these close together, bright planets in a non-urban area, without interference by electric wires and cables, so decided to travel to the remote area of Arroyo de San Serván southwestern Spain. However, because a sirocco was ushering Saharan dust across the Mediterranean Sea it was hard to distinguish the horizon through the camera's viewfinder (top photo), even though the solar disk was readily apparent. As darkness fell, I was surprised that I could discern both planets (bottom photo -- Venus is at right). Nonetheless, moments later seeing them was much more challenging.
Since path length is greater near the horizon than overhead (zenith) light scattering is more severe -- the sky is less transparent as the horizon is approached. Thus, celestial objects when viewed close to the horizon aren't as sharp as when viewed overhead. This is true even when the sky is clear, but of course, if the sky is dusty, seeing conditions are greatly diminished.
Photo Details: Equiv 640 mm lens; 1/250 sec. exposure; f/5.6; ISO 100.